A yawn (synonyms chasma, pandiculation[1], oscitation from the Latin verb oscitare, to open the mouth and have bad smells come out[2]) is a reflex of deep inhalation and exhalation associated with tiredness, stress, overwork, lack of stimulation, or boredom. Pandiculation is the term for the act of stretching and yawning.[1] Yawning is a powerful non-verbal message with several possible meanings, depending on the circumstances.
The claim that yawning is caused by lack of oxygen has not been substantiated scientifically.[3] However, the exact causes of yawning are still undetermined.
The word "yawn" has evolved from the Middle English word yanen, an alteration of yonen or yenen, which in turn comes from the Old Englishgeonian.[4]

A recent hypothesis raised in 2007 by Andrew C. Gallup and Gordon Gallup of the University of Albany states that yawning may be a means to keep the brain cool. Mammalian brains operate best when they are cool. In an experiment, he showed several groups of people videos of other people yawning. When the subjects held heat packs up to their foreheads while viewing the videos, they yawned often. But when they held cold packs up to their foreheads or breathed through their noses (another means of brain cooling), they did not yawn at all. [5][6] A similar recent hypothesis is that yawning is used for regulation of body temperature.

Another hypothesis is that yawns are caused by the same chemicals (neurotransmitters) in the brain that affect emotions, mood, appetite, and other phenomena. These chemicals include serotonin, dopamine, glutamic acid, and nitric oxide. As more (or less) of these compounds are activated in the brain, the frequency of yawning increases. Conversely, a greater presence in the brain of opiateneurotransmitters such as endorphins reduces the frequency of yawning. Patients taking the selective serotonin reuptake inhibitors Paxil (paroxetine HCl) or Celexa (citalopram) have been observed yawning more often. Excessive yawning is more common during the first three months of taking the SSRI's. Anecdotal reports by users of psilocybin mushrooms often describe a marked stimulation of yawning while intoxicated, often associated with excess lacrimation and nasal mucosal stimulation, especially while "peaking" (i.e., undergoing the most intense portion of the psilocybin experience). While opioids have been demonstrated to reduce this yawning and lacrimation provoked by psilocybin, it is not clear that the same pathways that induce yawning as a symptom of opioid abstinence in habituated users are the mode of action in psilocybin-induced yawning. While even opioid-dependent users of psilocybin on stable opioid therapy often report yawning and excess lacrimation while undergoing this entheogenic mushroom experience, there are no known reports in the literature that suggest psilocybin acts as any sort of general opioid antagonist. Psilocybin-induced yawning in opioid-habituated users does not appear to produce other typical opioid withdrawal symptoms such as cramping, physical pain, anxiety, gooseflesh, etc.

Recent research carried out by Catriona Morrison, a lecturer in psychology at the University of Leeds, involving monitoring the yawning behaviour of students kept waiting in a reception area, indicates a connection (supported by neuro-imaging research) between empathic ability and yawning. "We believe that contagious yawning indicates empathy. It indicates an appreciation of other people's behavioural and physiological state," said Morrison.[7]

Another theory is that yawning is similar to stretching. Stretching, like yawning, increases blood pressure and heart rate while also flexing many muscles and joints. It is also theorized that yawning helps redistribute surfactant, an oil-like substance which coats the lungs and aids breathing. Some have observed that if one tries to stifle or prevent a yawn by clenching one's jaws shut, the yawn is unsatisfying. As such, the stretching of jaw and face muscles seems to be necessary for a satisfactory yawn.

Yet another theory is that yawning occurs to stabilize pressure on either side of the ear drums. The deep intake of air can sometimes cause a popping sound that only the yawner can hear; this is the pressure on the middle ear stabilizing. This commonly occurs in environments where pressure is changing relatively rapidly, such as inside an airplane and when travelling up and down hills, which cause the eardrums to be bent instead of flat. Some people yawn when storms approach, which is a sure sign that changes in pressure affect them.

The yawn reflex is often described as contagious: if one person yawns, this will cause another person to "sympathetically" yawn.[3][8] Observing another person's yawning face (especially his/her eyes), or even reading about or thinking about yawning, can cause a person to yawn.[3][9] However, only about 55% of people in a given audience will respond to such a stimulus; fewer if only the mouth is shown in a visual stimulus.[10]The proximate cause for contagious yawning may lie with mirror neurons, i.e., neurons in the frontal cortex of certain vertebrates, which upon being exposed to a stimulus from conspecific (same species) and occasionally interspecific organisms, activates the same regions in the brain.[11] Mirror neurons have been proposed as a driving force for imitation which lies at the root of much human learning, e.g., language acquisition. Yawning may be an offshoot of the same imitative impulse.
A 2007 study found that children with autism spectrum disorders, unlike typical children, did not yawn after seeing videos of other people yawning; this supports the claim that contagious yawning is based on the capacity for empathy.[12]

To look at the issue in terms of evolutionary advantage, if there is one at all, yawning might be a herd instinct.[13] Other theories suggest that the yawn serves to synchronize mood behavior among gregarious animals, similar to the howling of the wolf pack. It signals tiredness to other members of the group in order to synchronize sleeping patterns and periods of activity. This phenomenon has been observed among various primates. The threat gesture is a way of maintaining order in the primates' social structure. Specific studies were conducted on chimpanzees[14] and stumptail macaques[15]. A group of these animals was shown a video of other conspecifics yawning, and both chimpanzees and stumptail macaques yawned also. This helps to partly confirm a yawn's "contagiousness."

Gordon Gallup, who hypothesizes that yawning may be a means of keeping the brain cool, also hypothesizes that "contagious" yawning may be a survival instinct inherited from our evolutionary past. "During human evolutionary history when we were subject to predation and attacks by other groups, if everybody yawns in response to seeing someone yawn, the whole group becomes much more vigilant, and much better at being able to detect danger."[5]

Certain superstitions surround the act of yawning. The most common of these is the belief that it is necessary to cover one's mouth when one is yawning in order to prevent one's soul from escaping the body. The Ancient Greeks believed that yawning was not a sign of boredom, but that a person's soul was trying to escape from its body, so that it may rest with the gods in the skies. This belief was also shared by the Maya.[How to reference and link to summary or text]

Other superstitions include:

A yawn is a sign that danger is near.

Counting a person's teeth robs them of one year of life for every tooth counted. This is why some people cover their mouths when they laugh, smile, or yawn.

If two persons are seen to yawn one after the other, it is said that the one who yawned last bears no malice towards the one who yawned first.

The one who yawns first shows no malice towards those he or she yawns around.

If you don't cover your mouth while yawning, then the devil will come and steal your soul (Estonia).

In some Latin American, East Asian and Central African countries yawning is said to be caused by someone else talking about you.

A yawn may be a sign that one is afflicted by the evil eye (Greece).

When one person yawns, it is said that anybody watching will instantly yawn as well

These superstitions may not only have arisen to prevent people from committing the faux pas of yawning loudly in another's presence — one of Mason Cooley'saphorisms is "A yawn is more disconcerting than a contradiction" — but may also have arisen from concerns over public health. Polydore Vergil (c. 1470–1555), in his De Rerum Inventoribus, writes that it was customary to make the sign of the cross over one's mouth, since "alike deadly plague was sometime in yawning, wherefore men used to fence themselves with the sign of the cross...which custom we retain at this day."[17]

Delini-Stula, A., & Hunn, C. (1990). Effects of single and repeated treatment with antidepressants on apomorphine-induced yawning in the rat: The implication of !a-1 adrenergic mechanisms in the D-2 receptor function: Psychopharmacology Vol 101(1) May 1990, 62-66.

Deputte, B. L. (1994). Ethological study of yawning in primates: I. Quantitative analysis and study of causation in two species of Old World monkeys (Cercocebus albigena and Macaca fascicularis): Ethology Vol 98(3-4) Nov-Dec 1994, 221-245.